Charge pump device with reduced ripple and spurious low frequency electromagnetic signals

ABSTRACT

Charge pump circuit providing a boosted output voltage (Vout) from an input DC voltage (Vin) and including a first capacitor ( 1 ) to which the input voltage is periodically applied for a determined first time (φ′ 1 ) by means of switch elements ( 3 A,  3 B) and another capacitor ( 2 ) to which the charge from the first capacitor is periodically transmitted by means of switch elements ( 4 A,  4 B) switched on for a determined second time (φ′ 2 ), said another capacitor being connected, on one side, to a terminal receiving the input voltage and, on the other side, to a terminal providing the boosted output voltage. 
     A voltage and/or current control device includes means ( 9, 3 C,  4 C) controlling the charge transmitted to the first capacitor during first times according to a determined reference voltage (Vref) and the charge transmitted from the first capacitor to the second during second times according to the boosted output voltage.

The invention relates to charge pump circuits, as for instance found involtage regulators and power supplies, and more particularly to avoltage and/or current control device for a charge pump circuit.

Charge pump circuits are used to provide boosted voltages from lowerinput voltages in integrated circuits when there is a need for such aboosted voltage to feed as required an internal sub-circuit and/or acoupled external circuit requiring a voltage which is higher than thesupply voltage which is supplied otherwise.

In a known solution, the output supply obtained from a charge pumpcircuit is submitted to an on-off regulation triggered from a comparatorcircuit. This comparator circuit controls charge pump circuit switchelements in order to have a required boosted voltage provided at anoutput of the charge pump circuit, as long as the voltage at this outputdoes not exceed a determined first threshold value. It also controls thecharge pump circuit switch elements in order to have a high impedance atthe charge pump circuit output when the output voltage exceeds the firstthreshold value and as long as this output voltage remains higher than asecond threshold value, smaller than the first threshold value. Such anon-off regulation involving successive switching operations generatesspurious signals and more particularly spurious low frequencyelectromagnetic signals. Furthermore hysteresis is associated with theuse of two voltage threshold values and it induces spurious outputvoltage ripples.

An object of this invention, therefore, is to provide an improved chargepump circuit supplying a boosted output voltage from an input DCvoltage. Such a circuit includes capacitors with a first capacitor towhich the input voltage is periodically applied for a determined firsttime by means of switch elements of a first switch module and anothercapacitor to which the charge from the first capacitor is periodicallytransmitted by means of switch elements of a second switch module whichare switched on for a determined second time which is included betweentwo successive first times. Said another capacitor is connected, on oneside, to an input terminal receiving the input voltage and, on the otherside, to an output terminal providing the boosted output voltage.

According to the invention, the charge pump circuit comprises a controldevice for controlling the charge transmitted to the first capacitorduring said first times according to a determined reference voltage andthe charge transmitted from the first capacitor to the second capacitorduring said second times according to the voltage obtained at the outputterminal.

An embodiment of a charge pump circuit according to the invention,comprises a control device including a common control means forcontrolling the charges respectively transmitted to the first capacitorand between the first and the other capacitor, said means beingactivated under a control according to the reference voltage during saidfirst times and under a control according to the boosted output voltageduring said second times.

Common control means are preferably inserted between the input terminalreceiving the input voltage and a junction point which is common to twoswitch elements respectively connected each on a different side ofcapacitor which are alternatively switched on either during first timesfor one or during second times for the other.

According to a preferred embodiment, common control means corresponds toa transistor having a gate linked by a further switch element of thefirst switch module to a terminal providing said reference voltage andby another switch element of the second switch module to the outputterminal.

These and other objects, advantages and features of the invention willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings below listed.

FIG. 1 illustrates an example of a charge pump circuit according toprior art.

FIG. 2 illustrates a charge pump circuit according to the invention.

As indicated above FIG. 1 relates to a known charge pump circuit 1 asfound for instance in a power supply or a voltage regulator for boostinga DC input voltage Vin to an appropriate upper DC voltage level Voutrequired by an user circuit operated in relation with the input voltageVin. The charge pump circuit comprises two capacitors 1 and 2 associatedwith two switch modules made for instance of NMOS elements, each moduleincluding two simultaneously controlled switch elements referenced 3A,3B for a first of the two switch modules and 4A, 4B for the secondswitch module.

Capacitor 1 is located between the two switch elements 3A, 3B withswitch element 3A connected to an input terminal of the charge pumpcircuit through which input voltage Vin is applied and with switchelement 3B connected to a ground terminal. Accordingly capacitor 1 ischarged at input voltage Vin as soon as the two switch elements 3A, 3Bare switched on during a time φ1 corresponding to a voltage input pulselong enough to restore a determined charge during a one pulse boostingphase, such boosting phases being usually subsequent to an initialcharging phase during which capacitor 1 is charged from scratch.

Capacitor 2 is located between the input terminal receiving voltage Vinand an output terminal of the charge pump circuit through which outputvoltage Vout is provided and also between the two switch elements 4A,4B. Switch element 4A is connected on one side to the input terminal ofthe charge pump circuit and on the other side to a junction pointlocated between capacitor 1 and switch element 3B. Switch element 4B isconnected on one side to the output terminal of the charge pump circuitand on the other side to a junction point located between capacitor 1and switch element 3A. The charge stored in capacitor 1 is transferredto buffer capacitor 2, when switch elements 4A and 4B are switched onduring a time φ2 during which switch elements 3A and 3B are switchedoff. Time φ2 corresponds to a voltage pulse long enough to restore thecharge of capacitor 2 to a predetermined level at the end of a one pulseboosting phase.

Control means 5 are associated to switch elements 3A, 3B and 4A, 4B,that they drive under control of a triggering voltage logic based on acomparator 6 linked to the output terminal of the charge pump circuitthrough a voltage dividing bridge. Comparator 6 is connected by a firstinput to a junction point common to two resistors 7 and 8 of the voltagedividing bridge, with resistor 7 connected to the output terminal of thecharge pump circuit and resistor 8 connected to the ground terminal.

Two threshold voltages Vref1 and Vref2 are applied to two second inputsof comparator 6 for comparison purposes with the image of the outputvoltage of the pump charge circuit which is obtained through the voltagedividing bridge. Output voltage Vout corresponds to the sum of inputvoltage Vin with half of the sum of Vref1 with Vref2.

When the voltage at the first input of comparator 6 exceeds Vref1 theoutput of the pump charge circuit is driven to a high impedance andremains at this high impedance as long as the output voltage remainsabove voltage threshold Vref2 which is lower than Vref1.

After an initial charging phase, a recharge of capacitor 1 isperiodically insured as soon as switch elements 3A and 3B aresimultaneously and periodically switched on by control means 5 whichreceives pulse signals φ1 from a timer. Switch elements 4A and 4B aresimultaneously and periodically switched on by control means 5 whenswitch elements 3A, 3B are switched off which receives pulse signals φ2from the timer, with φ2 as provided by the timer corresponding at leastapproximately to the inverted signal of φ1.

A transfer of charge from capacitor 1 to capacitor 2 occurs during timesφ2, such a transfer is stopped by control means 5 under control ofcomparator 6, as soon as a voltage higher than Vref1 is applied to thecomparator by means of the dividing bridge

As previously indicated, unwanted low frequency signals are generateddue to the on-off regulation. Voltage ripple is caused by the hysteresisassociated to this on-off regulation in relation with the variations ofthe output voltage between the two thresholds Vref1 and Vref2.

An improvement of such a charge pump circuit is shown on FIG. 2. Samereferences are used for the elements common to the prior art solutionshown on FIG. 1 and to the circuit according to the invention.

Capacitor 1 remains located between two switch elements 3A, 3B of a samefirst switch module having three elements, with switch element 3Bconnected to a ground terminal. Switch element 3A is connected to theinput terminal of the charge pump circuit by means of a device 9. Thisdevice is for instance a transistor such as a NMOS FET chosen for itsratio size/efficiency.

Capacitor 2 remains located between the input terminal receiving voltageVin and the output terminal of the charge pump circuit through whichoutput voltage Vout is provided. Switch element 4A is a part of a secondswitch module having three elements, it is again connected on one sideto a junction point located between capacitor 1 and switch element 3B.It is connected on its other side to a junction point located betweendevice 9 and switch element 3A. Switch element 4B which belongs to thesaid second switch module remains connected on one side to the outputterminal of the charge pump circuit and to a junction point locatedbetween capacitor 1 and switch element 3A, on the other side.

According to the invention, a voltage reference Vref, corresponding to athreshold loss, is applied to the gate of device 9 by means of a thirdswitch element 3C of the first switch module when switch elements 3A, 3Bare switched on for a time φ1, as defined above. The gate of device 9 isalso linked to the output of the charge pump circuit by means of a thirdswitch element 4C of the second switch module.

Capacitor 1 is initially charged, then recharged, as soon as switchelements 3A, 3B, 3C are switched on by periodic pulse signals φ′1 from atimer 10 which also delivers pulse signals φ′2 corresponding broadly topulse signals φ1 and φ2. During φ′1, the input voltage Vin provided tothe input terminal of the pump charge circuit, which is for instance a20 volts DC voltage, is then applied to capacitor 1 through device 9.This device 9 then controls the current circulating through switchelement 3A in accordance with reference voltage Vref, which is forinstance a 10 volts DC voltage. In a preferred embodiment, an additionalvoltage Vt is added to voltage Vref at the gate of device 9 throughswitch element 3C to compensate for the threshold of device 9.

A transfer of charge from capacitor 1 to capacitor 2 occurs during timesφ′2, through switch elements 4A, 4B and the output voltage Vout obtainedat the output terminal of the charge pump circuit is applied to the gateof device 9 by means of switch element 4C.

The output voltage that is obtained corresponds to the sum of voltagesVin and Vref.

The suppression of the comparator with two voltage references, as foundin the prior art, reduces the ripple and the spurious low frequencyelectromagnetic signals. Inrush current is also limited due to the linkestablished between the output terminal of the charge pump circuit andthe gate of device 9 through switch element 4C.

What is claimed is:
 1. A charge pump circuit providing a boosted output voltage from an input DC voltage and comprising: a first capacitor to which the input voltage is periodically applied for a determined first time by means of first switch elements of a first switch module, and a second capacitor to which the charge from the first capacitor is periodically transmitted by means of second switch elements of a second switch module which are switched on for a determined second time which is included between two successive first determined times, said second capacitor having a first terminal connected to an input terminal receiving the input voltage and a second terminal connected and, on the other side, to an output terminal providing the boosted output voltage said circuit comprising a control device including a common control means for controlling the charge transmitted to the first capacitor during said first times according to a determined reference voltage and the charge transmitted from the first capacitor to the second capacitor during said second times according to the boosted output voltage obtained at the output terminal, wherein said common control means is a device having a control electrode linked by a switch element of the first switch module to a terminal providing said reference voltage and by a switch element of the second switch module to the output terminal.
 2. The charge pump circuit according to claim 1, wherein said common control means is activated under a control according to said reference voltage during said first times and under a control according to the boosted output voltage during said second times.
 3. A charge pump circuit according to claim 2, wherein said common control means are inserted between the input terminal receiving the input voltage and a junction point which is common to one of the first switch elements and one of the second switch elements respectively, each connected on a different side of the first capacitor, which are alternatively switched on either during first times for one of the first switch elements or during second times for one of the second switch elements.
 4. A charge pump circuit according to claim 1, wherein the common control means comprises a transistor, wherein the control electrode is a gate of the transistor. 